Highly conducting and transparent thin polymer films formed from double and multiple layers of poly(3,4-ethylenedioxythiopene) and its derivatives

ABSTRACT

A polymer film comprising at least two layers, wherein each layer comprises a compound comprising the formula:                  
 
wherein R 1  and R 2  are independently selected organic groups. A method of making a polymer film comprising the steps of: providing a monomer solution comprising one or more monomers comprising the formula:                  
 
wherein R 1  and R 2  are independently selected organic groups; dispensing the monomer solution onto a substrate; heating the monomer solution on the substrate to polymerize the monomer; and repeating the steps of providing a monomer solution, dispensing, and heating one or more times, wherein the spin-coating is performed on top of the prior spin-coated layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to multilayer conductive polymer filmscomprising a poly(3,4-ethylenedioxythiophene).

2. Description of the Prior Art

Conducting polymers can be used in organic light-emitting diodes(OLED's). OLED's are an attractive alternative to liquid crystal displaytechnology because they can yield displays that are brighter, lowercost, consume less power, and are lightweight. Other examples ofapplications for conducting polymers are anticorrosion coatings andprinted circuit board finishes.

Extended π-conjugated conducting oligomers and polymers have uniqueproperties that have impacted diverse technologies, and have resulted inthe appearance of new ones. A partial list of current and developingapplications includes micro- and nanoscale circuitry, throwawayelectronic devices such as plastic electrochromic displays, lightweightstorage batteries, corrosion protection coatings, antistatic coatings,bio- and chemical sensors, and military applications such asmicrowave-absorbing materials. In all of these applications a highdegree of polymer transparency in visible wavelengths is eithernecessary or could represent an additional advantageous trait.

Key properties of π-conjugated conducting oligomers and polymers such asbandgap, dielectric constant, and oxidation potential can be varied overmuch wider ranges than those of other transparent inorganic conductorssuch as indium tin oxide (ITO) ceramic. This is because of the vastdiversity inherent to the organic chemistry of π-conjugated monomers.Other advantages over metals and inorganics include greater plasticityand elasticity, lower mass density, lower coefficient of thermalexpansion, greater resistance to chemicals and corrosion,electrochromism, and enhanced power storage capabilities.

As an example of a specific application wherein a highly transparentconducting polymer could have a large impact, one can consider theliquid crystal display devices (LCD's) that are extremely important incurrent information technology and OLED's under development for nextgeneration displays. In these devices, or in any display device,transparent electrodes are a prime requirement and ITO coated on glassor clear plastic surfaces has generally been used because of its hightransparency (˜90%), low surface resistance (˜70 ohms/sq), and highconductivity (˜1000 S/cm). However, the technology is quite expensiveand requires high temperature and vacuum treatment. Moreover, thebrittleness of the ITO, the non-stoichiometric nature of ITO surfaces,and poor adhesion at the inorganic-organic interface causes seriousproblems. The deposition of transparent, conductive polymer film onplastic substrates is a highly promising alternative that allowscircumvention of these problems. FIGS. 1 and 2 schematically illustrateembodiments of a LCD (FIG. 1) and an OLED (FIG. 2).

Because conducting oligomers/polymers are highly conjugated, they may becolored both in the neutral undoped (non-conducting) state as well as inthe cationic, doped (conducting) state. The development of highlytransparent conducting polymer thin films has therefore beenchallenging. Prior art has centered on three families of conductingpolymers, polyaniline (PANI), polypyrrole (PPY), and poly(3,4alkylenethiophene) (PATP).

Jonas et al., U.S. Pat. No. 5,035,926 discloses single layer coatings ofpoly(3,4-dioxythiophene) made by surface polymerization.

U.S. Pat. No. 6,327,070 to Heuer et al. discloses PATP's with alkylidenegroups such as ethylene, propylene, and butylene as well as thosecontaining phenyl and tetradecyl moieties yielding films with modestproperties when formed via electropolymerization. For example, poly(3,4ethylenedioxythiophene) films had a conductivity of 8 S/cm with atransparency of 52%.

An attractive attribute of the monomeric alkylidenethiophenes is theirlow oxidation potential (˜0.4 V relative to Ag/AgCl) that allows use ofmild oxidation agents and results in polymer with high chemicalstability. The polymers also have a low band gap (1.5–1.6 eV), causingtheir absorption λ_(max) values to appear at relatively long wavelengths(590 nm for the undoped form and 775 nm for the doped form). Thecorresponding colors are dark violet and sky blue. The absorption in thedoped conducting form is shifted into the infrared region and thereforethe polymers become less heavily colored and are more transparent to thehuman eye. Within this class of conducting polymers, by far the mostextensively investigated has been poly(3,4 ethylenedioxythiophene), orPEDOT, the simplest one from the standpoint of chemical structure. FIG.3 illustrates the reaction scheme for the polymerization of PEDOT intoboth doped and undoped forms.

A polymerization method that is well suited for monomers with lowoxidation potential such as PEDOT utilizes an oxidant, a base, and analcohol solvent oxidant. At moderately high temperatures (˜100° C.) thepolymerization occurs very rapidly. De Leeuw et al., “Electroplating ofConductive Polymers for the Metallization of Insulators,” Synth. Metals,66(3), 263 discloses that if the reactant-containing solution isspin-coated onto a suitable substrate such as plastic or glass and thenheated, highly conducting insoluble sky-blue films are formed. FIG. 4illustrates the reaction scheme for the polymerization of PEDOT bysurface polymerization. See also Kumar et al., “ConductingPoly(3,4-alkylenedioxythiophene) Derivatives as Fast Electrochromicswith High-Contrast Ratios,” Chem. Mater., 10(3), 896 and Pei et al.,“Electrochromic and Highly Stable Poly(3,4-ethylenedioxythiophene)Switches Between Opaque Blue-black and Transparent Sky Blue”, Polymer,35(7), 1347–1351.

SUMMARY OF THE INVENTION

The invention comprises a conductive polymer film comprising at leasttwo layers, wherein each layer comprises a compound comprising Formula(1):

wherein R¹ and R² are independently selected organic groups.

The invention further comprises a method of making a conductive polymerfilm comprising the steps of: providing a monomer solution comprisingone or more monomers comprising Formula (2):

wherein R¹ and R² are independently selected organic groups; dispensingthe monomer solution onto a substrate; heating the monomer solution onthe substrate to polymerize the monomer; and repeating the steps ofproviding a monomer solution, spin-coating, and heating one or moretimes, wherein the spin-coating is performed on top of the priorspin-coated layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic illustration of the construction of an embodiment ofa LCD device;

FIG. 2 is schematic illustration of the construction of an embodiment ofan OLED device;

FIG. 3 illustrates the reaction scheme for the polymerization of PEDOTinto both doped and undoped forms;

FIG. 4 illustrates the reaction scheme for the polymerization of PEDOTby surface polymerization;

FIG. 5 is a graph comparing the properties of single and double layeredfilms as made in Example 1;

FIG. 6 is a graph comparing the properties of single and double layeredfilms as made in Example 2;

FIG. 7 is a graph comparing the properties of single and double layeredfilms as made in Example 3;

FIG. 8 is a graph comparing the properties of single and double layeredfilms as made in Example 4;

FIG. 9 is a graph comparing the properties of single and double layeredfilms as made in Example 5;

FIG. 10 is a graph comparing the properties of single and double layeredfilms as made in Example 6;

FIG. 11 is a graph comparing the properties of single and double layeredfilms as made in Example 6;

FIG. 12 is a graph comparing the properties of single and double layeredfilms as made in Example 6;

FIG. 13 is a graph comparing the properties of single and double layeredfilms as made in Example 6;

FIG. 14 is a graph comparing the properties of single and double layeredfilms as made in Example 6;

FIG. 15 is a graph comparing the properties of single and double layeredfilms as made in Example 7; and

FIG. 16 is a graph comparing the properties of single and double layeredfilms as made in Example 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention comprises a method of forming highly conductingtransparent polymer films based on poly(3,4 ethylenedioxythiophene) andits derivatives. In this spin-coating procedure, more than one thinlayer is sequentially polymerized on a substrate. Key polymer propertiessuch as conductivity, low surface resistance, and optical transparencymay be improved in this manner. Two layered thin films with a givenadditive thickness may possess a higher bulk conductivity, a lowersurface resistance, and a transparency equal to or greater than that ofa single layer film of the same thickness. In some instances, this canbe the case even if the total thickness of the double-layer film is lessthan that of the thick single-layer film. For example, single-layeredpoly(3,4 ethylenedioxythiophene) (PEDOT) with a film thickness of 0.2microns may have a surface resistance of 350 ohms and a 75% transparencyin the visible regime, but double-layered PEDOT with a film thickness of0.15 microns may have a significantly lower surface resistance of 280ohms and a slightly higher transparency of 77%. This behavior may beespecially pronounced if perfluoroalkyl-derivatized PEDOT is used inplace of normal PEDOT. Using the double- or multiple-layering method,films with very high conductivities (716 S/cm) with concomitant hightransparencies (82%) have been formed.

The method of the invention comprises the steps of providing a monomersolution, spin-coating, heating, and repeating. The monomer solutioncomprises one or more monomers comprising Formula (3):

wherein R¹ and R² are independently selected organic groups. As usedherein, the term “organic groups” includes hydrogen and hydroxyl. Themonomer solution may comprise a plurality of monomers or a singlemonomer. R¹ and R² may be independently selected from the groupconsisting of hydrogen, alkyl, alkenyl, alkynyl, aromatic, ether, ester,hydroxyl, amine, thiol, thione, sulfide, sulfonate, phosphine,phosphate, and phosphonate.

In some embodiments, both R¹ and R² are hydrogen so that the monomer is3,4-ethylenedioxythiophene. In some embodiments, R² is an ester suchthat the monomer comprises Formula (4):

wherein R³ and R⁴ are independently selected organic groups. In thismonomer, R¹ and R⁴ may both be hydrogen. R³ may be a fluorinated groupselected from the group consisting of alkyl, linear alkyl having from 1to 14 carbon atoms, aromatic, cycloaliphatic, carbohydrate, amine,ketone, ether, alkenyl, alkynyl, secondary amine, tertiary amine,thione, sulfide, sulfonate, sulfate, phosphine, phosphate, andphosphonate. Suitable R³ groups include, but are not limited to,perfluoroalkyl, 1,1,2,2,3,3,4,4,4-nonafluorobutyl, and1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecylfluorooctyl. The latter twomonomers are shown in Formula (5).

The monomer solution can also comprise an oxidant, a base, and analcohol solvent. This mixture may catalyze the polymerization of themonomer. Iron (III) p-toluenesulfonate is a suitable oxidant andimidazole is a suitable base. Suitable alcohol solvents include, but arenot limited to, alcohols having from one to five carbon atoms,1-butanol, 2-propanol, and 2-methoxyethanol.

In the dispensing step, the monomer solution is dispensed onto asubstrate. This may be done by spin-coating. The substrate may comprise,but is not limited to, a plastic, a glass, a metal, and/or a ceramic.The quantity of the solution and the speed of spinning may be selectedsuch that the resulting polymer layer is no more than about 0.5 μmthick.

In the heating step, the spin-coated monomer solution is heated topolymerize the monomer. Suitable heating conditions include, but are notlimited to, a temperature of from about 80 to about 120° C. and a timeof no more than about 3 minutes. The polymer layer may also be washedand/or dried. Optimization of the heating parameters is within theordinary skill in the art. The process of spin-coating and heating isreferred to herein as surface polymerization.

In the repeating step, the steps of providing a monomer solution,dispensing, and heating are repeated one or more times. Each successivelayer is placed on top of the previous layer to build up a multilayerpolymer film. The same monomer solution can be used in each repetition,or different monomer solutions can be used. There is no limit to thenumber of times the repeating step may be performed. However, one tothree and one to nine repetitions are suitable.

The invention also comprises a polymer film comprising at least twolayers, wherein each layer comprises a compound comprising Formula (1).Formula (1) may be a homopolymer or may have more than one monomer. TheR¹ and R² substituents are as described above. The film may be made bythe method described above, although the invention is not limited tofilms made by this method. There is no limit to the number of layers inthe film. However, two to four and two is ten layers are suitable.

The polymer film may have high conductivity and high transparency. Thefilm may have a conductivity of at least about 100 S/cm and atransparency of at least about 80%. These properties can make thepolymer film useful in display devices. A display device can befabricated from the polymer film by methods known in the art.

Surface polymerization can produce films with low surface resistance.However, single-layer films may not possess acceptable combinations oftwo major properties crucial for display application, i.e. low filmsurface resistance and high optical transparency. Typical values forsingle-layer films are in the range 200–350 ohms and 50–75 percenttransparency in the visible range. The surface resistance (SR) valuescorrespond to conductivity values of 170 and 100 S/cm, respectively(conductivity=1/(SR·film thickness)). By optimizing various processparameters, molar ratios between components, spinning speed,polymerization temperature, polymerization time, reaction solvent, andtotal concentration of solution the surface resistance may be maintainedin the above range while raising the optical transparency into the rangeof 60–85 percent in a single-layer film. However, this optimizationprocedure may not produce a desired combination of 100 ohms (500 S/cm)and 85% T.

However, the double- or multiple-layer coating technique can produce afilm with a conductivity of 714 S/cm and an optical transparency of 82%.Films constructed using this approach can have lower surface resistanceand higher transparencies than their single-layer counterparts. Thedouble- or multiple-layer approach thus allows these two key filmproperties to be correlated in a positive manner. Derivatized EDOT canyield films with conductivities and transparencies superior to thoseformed from underivatized EDOT.

For the polymerization of 3,4-ethylenedioxythiophene and itsperfluoroalkyl derivatives, the advantages of the double- and multilayermethods are that they may lead to the formation of thin conductingpolymer films with maximized desirable properties such as highconductivity, low surface resistance, and high optical transparency. Twolayered thin films with a given additive thickness can possess a higherbulk conductivity, a lower surface resistance, and a transparency equalto or greater than that of a single layer film of the same thickness.

Film properties may be optimal if the number of layers is increased toup to ten while using faster spin-coating speeds to attain thinnerfilms. Also, the double- or multilayering process can be employed forthe polymerization of other derivatives of 3,4 ethylenedioxythiophenesuch as those containing an alkane, alkene, alkyne, or aromatic group,or an ether, hydroxyl, amine, thiol, thione, sulfide, sulfonate,sulfate, phosphine, phosphate, or phosphonate.

Having described the invention, the following examples are given toillustrate specific applications of the invention. These specificexamples do not limit the scope of the invention described in thisapplication.

Comparison of Single vs. Double-Layered Films

EXAMPLE 1

Formation of single- and double-layered thin films of poly(3,4ethylenedioxythiophene) and comparison of properties—The results forExample 1 are given in FIG. 5. The polymer film was synthesized using anEDOT monomer solution in 3 mL 1-butanol that contained 0.85 M iron (III)tosylate, 0.66 M imidazole, and 0.33 M EDOT. The total soluteconcentration was 40% by weight.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 3000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be350 ohms/square and 75% (averaged over the range 350–750 nm),respectively. The film thickness was measured gravimetrically and foundto be 0.2±0.03 μm.

For the double coating, the monomer solution was spin-coated using aspin speed of 8000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 10 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. Additional monomer solution (1.0 mL) waspipetted onto the polymer film and spin-coated, again using a spin speedof 8000 RPM. The double-layered film was rinsed with methanol, driedunder nitrogen gas as above, and its surface resistance and transparencywere quantified and found to be 280 ohms/square and 77% (averaged overthe range 350–750 nm), respectively. The thickness of the double filmwas measured gravimetrically and found to be 0.15±0.03 μm.

Comparison of properties shows that the double-layered film has asurface resistance 20% lower than that of the single-layer film, and hasa slightly higher transparency. Furthermore, the total thickness of thedouble film is 25% less than that of the single film. Therefore, thelayering method yields films that have superior properties even thoughthey are thinner than their single-layer counterpart.

EXAMPLE 2

Formation of single- and double-layered thin films of poly(3,4ethylenedioxythiophene) and comparison of properties—The results forExample 2 are given in FIG. 6. The polymer film was synthesized using anEDOT monomer solution in 3 nL 1-butanol that contained 0.66 M iron (III)tosylate, 0.66 M imidazole, and 0.33 M EDOT. The total soluteconcentration was 37.5%.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 1000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be220 ohms/square and 61% (averaged over the range 350–750 nm),respectively. The film thickness was measured gravimetrically and foundto be 0.25±0.03 μm.

For the double coating, the monomer solution was spin-coated using aspin speed of 8000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 3 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. Additional monomer solution (1.0 mL) waspipetted onto the polymer film and spin-coated, again using a spin speedof 8000 RPM. The double-layered film was rinsed with methanol, driedunder nitrogen gas as above, and its surface resistance and transparencywere quantified and found to be 210 ohms/square and 82% (averaged overthe range 350–750 nm), respectively. The thickness of the double filmwas measured gravimetrically and found to be 0.15±0.03 μm.

Comparison of properties shows that the double-layered film has atransparency that is 26% higher than that of the single-layer film, andhas a surface resistance that is 5% lower. In this case, the totalthickness of the double film is 40% less than that of the single film.This is a dramatic example showing that the layering method yields filmsthat have superior properties even though their additive thickness isless than their single-layer counterpart.

EXAMPLE 3

Formation of single- and double-layered thin films ofpoly(2,2,3,3,4,4,5,5,5-nonafluoropentanoic acid2,3-dihydro-thieno[3,4-b][1,4]dioxin-2-ylmethyl ester 3,4ethylenedioxythiophene) and comparison ofproperties—2,3-Dihydro-thieno[3,4-b][1,4]dioxin-2-yl)-methanol(hereafter referred to as CH₂OH-EDOT) was esterified using2,2,3,3,4,4,5,5,5-nonafluoropentanoic acid and(1-ethyl-3-(3-dimethylaminopropyl)carbodiimide HCl) (EDC) catalyst inTHF. The product, 2,2,3,3,4,4,5,5,5-nonafluoropentanoic acid2,3-dihydro-thieno[3,4-b][1,4 ]dioxin-2-ylmethyl ester (hereafterreferred to as C₄F₉-EDOT, MW 418) was isolated in a 76% overall yieldand characterized using ¹H and ¹³C NMR, GC-MS, and elemental analysis.

The results for Example 3 are given in FIG. 7. The polymer films weresynthesized using a C₄F₉-EDOT monomer solution in 3 mL 2-methoxyethanolthat contained 0.58 M iron (III) tosylate, 0.66 M imidazole, and 0.33 MC₄F₉-EDOT. The total solute concentration was 35%.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 3000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be500 ohms/square and 94% (averaged over the range 350–750 nm),respectively. The film thickness was measured gravimetrically and foundto be 0.17±0.03 μm.

For the double coating, the monomer solution was spin-coated using aspin speed of 6000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 3 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. Additional monomer solution (1.0 mL) waspipetted onto the polymer film and spin-coated, again using a spin speedof 6000 RPM. The double-layered film was rinsed with methanol, driedunder nitrogen gas as above, and its surface resistance and transparencywere quantified and found to be 280 ohms/square and 92% (averaged overthe range 350–750 nm), respectively. The thickness of the double filmwas measured gravimetrically and found to be 0.17±0.03 μm.

Comparison of properties shows that the double-layered film has asurface resistance that is 44% lower than that of the single-layer film,but its transparency is only 2% less. Both film thicknesses are thesame. With regard to reduction in surface resistance, this example showsthat the multilayering strategy can be even more effective whenderivatized EDOT monomer is used vs. underivatized EDOT—the formerallowed a 44% reduction in resistance whereas the latter allowed only a25% reduction (i.e., Example 1).

EXAMPLE 4

Formation of single- and double-layered thin films ofpoly(C₄F₉-EDOT-co-CH₂OH-EDOT) and comparison of properties—The resultsfor Example 4 are given in FIG. 8. The polymer films were synthesizedusing an equimolar solution of C₄F₉-EDOT and CH₂OH-EDOT in 3 mL2-propanol that contained 0.58 M iron (III) tosylate, 0.66 M imidazole,and 0.33 M monomer mixture. The total solute concentration was 38%.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 3000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be85 ohms/square and 65% (averaged over the range 350–750 nm),respectively. The film thickness was measured gravimetrically and foundto be 0.25±0.04 μm.

For the double coating, the monomer solution was spin-coated using aspin speed of 6000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 3 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. Additional monomer solution (1.0 mL) waspipetted onto the polymer film and spin-coated, again using a spin speedof 6000 RPM. The double-layered film was rinsed with methanol, driedunder nitrogen gas as above, and its surface resistance and transparencywere quantified and found to be 50 ohms/square and 64% (averaged overthe range 350–750 nm), respectively. The thickness of the double filmwas measured gravimetrically and found to be 0.18±0.03 μm.

Comparison of properties shows that the double-layered film has asurface resistance 41% lower than that of the single-layer film, and hasa transparency that is only 1% less. In addition, the total thickness ofthe double film is 28% less than that of the single film. This isanother example of how the multilayering strategy can be even moreeffective in reducing surface resistance when derivatized EDOT monomeris used vs. underivatized EDOT. It also another example demonstratingthat the method can yield films that have superior properties eventhough their additive thickness is less than their single-layercounterpart.

EXAMPLE 5

Formation of single- and double-layered thin films of poly(C₄F₉-EDOT)and comparison of properties—The results for Example 5 are given in FIG.9. The polymer films were synthesized using a solution of C₄F₉-EDOT in2-methoxyethanol that contained 0.58 M iron (III) tosylate, 0.66 Mimidazole, and 0.33 M monomer. The total solute concentration was 35%.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 3000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be380 ohms/square and 92% (averaged over the range 350–750 nm),respectively. The film thickness was measured gravimetrically and foundto be 0.17±0.03 μm. The film conductivity was 155 S/cm.

For the double coating, the monomer solution was spin-coated using aspin speed of 6000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 3 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. Additional monomer solution (1.0 mL) waspipetted onto the polymer film and spin-coated, again using a spin speedof 6000 RPM. The double-layered film was rinsed with methanol, driedunder nitrogen gas as above, and its surface resistance and transparencywere quantified and found to be 280 ohms/square and 93% (averaged overthe range 350–750 nm), respectively. The thickness of the double filmwas measured gravimetrically and found to be 0.18±0.03 μm. The filmconductivity was 200 S/cm.

Comparison of properties shows that the double-layered film has asurface resistance 26% lower than that of the single-layer film, leadingto a conductivity that is 22% higher. The double-layered film has atransparency that is 1% greater than that of its single-layercounterpart.

EXAMPLE 6

Optimization of spin-coating conditions for various monomers—There aresix fundamental process variables for the surface polymerizationprocess—monomer concentration, molar ratio between solutes, choice ofalcohol solvent, spin-coating speed in RPM, polymerization temperature,and polymerization time. It was determined that the polymer films couldbe formed using various ranges of four process variables and fixedvalues or types of the other two (Table 1).

TABLE 1 Process variables for spin coating and polymerization of EDOT,C₄F₉-EDOT, and CH₂OH-EDOT Variable Range or type Total soluteconcentration 30–40 wt % Molar ratio—monomer:oxidant:base 1:2.6:2,1:2:2, 1:1.75:2, 1:1.5:2 Solvent butanol, 2-propanol, 2-methoxy- ethanolSpin speed 1000–8000 RPM Polymerization time 3 minutes Polymerizationtemperature 110° C.

Optimum conditions for the polymerization of each monomer (EDOT,C₄F₉-EDOT and CH₂OH-EDOT) were determined. It was found that the mostdesirable film properties, i.e. low surface resistance, highconductivity, and high transparency, were attained when the total soluteconcentration was held at about 40 wt %, the polymerization time held to3 min, and the polymerization temperature maintained at 110° C. Withinthese constraints the polymerization solvent, the spin-coating speed,and the molar ratio between monomer, oxidant and imidazole base wasvaried as shown in Table 1. It was found that n-butanol and2-methoxyethanol are the optimal solvents for the polymerization ofC₄F₉-EDOT, CH₂OH-EDOT, and EDOT. In addition, the optimal spin-coatingspeeds are at the high end of the range, 6000 and 8000 RPM. Finally, theoptimal solute molar ratios are 1:1.75:2 for C₄F₉-EDOT and CH₂OH-EDOT,and 1:2:2 for EDOT (see Table 1). This last variable may be the mostimportant—small changes in the ratio often led to large changes in filmproperties, with a heavy dependence on monomer type. This behavior issummarized in FIGS. 10 and 11. In these experiments, the spin speed washeld constant at 6000 RPM, double layers were formed, and n-butanol wasused as solvent. The films were formed in the manner described inExamples 1–5 with the solute molar ratios varying from 1:1.5:2 to1:2.6:2. The amount of oxidant was varied from 1.5 to 2.6 mmol. No PEDOTfilm was formed at 1.5 mmol oxidant. FIG. 10 shows surface resistanceand FIG. 11 shows transparency.

The results of the optimization are displayed in FIGS. 12 and 13. Inboth Figures the characteristics of spin-coated commercially availableconducting polymer (Baytron P®), are given in the left three columnsets. In the right three column sets, the characteristics of filmsformed from each monomer-specific optimized polymerization process aregiven. In all three cases, the films are comprised of double layersformed at 8000 RPM. The system giving the most attractive properties isthe C₄F₉-EDOT system, with a surface resistance of 160 ohms/sq, aconductivity of 716 S/cm, and an 82% transparency. The superiority ofthe double-layered films vs. the commercial systems is quite clear. FIG.14 shows the conductivities of the double-layered spin-coated films, thecommercially available material, and several families of metals,semi-conductors, and insulators. It is apparent that the double-layeringapproach allows the poly(C₄F₉-EDOT ) system to achieve metallicconductivity.

Multiple Coatings—Spin-Coating of PEDOT

In some applications, the film surface resistance is of greaterimportance than its bulk conductivity. The use of more than two thinfilms can yield drastically lower surface resistances while maintainingnearly constant film transparency.

EXAMPLE 7

Formation of single- and quadruple-layered thin films of poly(EDOT) andcomparison of properties—The results for Example 7 are given in FIG. 15.The polymer films were synthesized using a solution of EDOT in 4 mL1-butanol that contained 0.76 M iron (III) tosylate, 0.66 M imidazole,and 0.33 M monomer. The total solute concentration was 38%.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 2000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be580 ohms/square and 82% (averaged over the range 350–750 nm),respectively.

For the quadruple coating, the monomer solution was spin-coated using aspin speed of 4000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 3 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. This process was repeated three times, andthe film surface resistance and transparency were quantified and foundto be 250 ohms/square and 80% (averaged over the range 350–750 nm),respectively.

Comparison of properties shows that the quadruple-layered film has asurface resistance 57% lower than that of the single-layer film, withonly a 2% sacrifice in optical transparency.

EXAMPLE 8

Formation of single- and quadruple-layered thin films of poly(EDOT) andcomparison of properties—The results for Example 8 are given in FIG. 16.The polymer films were synthesized using a solution of EDOT in 4 mL2-propanol that contained 0.76 M iron (III) tosylate, 0.66 M imidazole,and 0.33 M monomer. The total solute concentration was 38%.

For the single coating, 1.0 mL of the monomer solution was pipetted ontoa clear plastic (polyethylene terepthalate) square substrate having athickness of 0.1 mm and a surface area of 6.5 cm². The substrate wasspin-coated using a spin speed of 2000 RPM. To form the polymer film,the substrate was immediately heated at atmospheric pressure to 110° C.for 3 min. It was then rinsed with methanol, dried with nitrogen gas andits surface resistance and transparency were quantified and found to be280 ohms/square and 55% (averaged over the range 350–750 nm),respectively.

For the quadruple coating, the monomer solution was spin-coated using aspin speed of 4000 RPM onto substrates identical to those describedabove, and polymerization was performed by heating the coated substrateto 110° C. for 3 minutes, as above. The film was rinsed with methanoland dried under nitrogen gas. This process was repeated three times, andthe film surface resistance and transparency were quantified and foundto be 130 ohms/square and 66% (averaged over the range 350–750 nm),respectively.

Comparison of properties shows that the double-layered film has asurface resistance 54% lower than that of the single-layer film, with an8% increase in optical transparency.

The co-filed U.S. Patent Application to Martin et al., “HighlyConducting Transparent Thin Films Formed from New FluorinatedDerivatives of 3,4-Ethylenedioxythiophene,” designated as Navy Case84,103 is incorporated herein by reference.

1. A polymer film comprising at least two layers, wherein each layercomprises a compound comprising the formula:

wherein R¹ and R² are independently selected organic groups; and whereinthe layers are formed by repeated surface polymerization on a substrate.2. The polymer film of claim 1, wherein the polymer is a homopolymer. 3.The polymer film of claim 1, wherein R¹ and R² are independentlyselected from the group consisting of hydrogen, alkyl, alkenyl, alkynyl,aromatic, ether, ester, hydroxyl, amine, thiol, thione, sulfide,sulfonate, phosphine, phosphate, and phosphonate.
 4. The polymer film ofclaim 1, wherein R¹ and R² are both hydrogen.
 5. The polymer film ofclaim 1, wherein the compound comprises the formula:

wherein R³ and R⁴ are independently selected organic groups.
 6. Thepolymer film of claim 1, wherein the compound comprises the formula:

wherein R³ is an organic group.
 7. The polymer film of claim 6, whereinR³ is a fluorinated group selected from the group consisting of alkyl,linear alkyl having from 1 to 14 carbon atoms, aromatic, cycloaliphatic,carbohydrate, amine, ketone, ether, alkenyl, alkynyl, secondary amine,tertiary amine, thione, sulfide, sulfonate, sulfate, phosphine,phosphate, and phosphonate.
 8. The polymer film of claim 6, wherein R³is perfluoroalkyl.
 9. The polymer film of claim 6, wherein R³ is1,1,2,2,3,3,4,4,4-nonafluorobutyl.
 10. The polymer film of claim 6,wherein R³ is 1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecylfluorooctyl.11. The polymer film of claim 1, wherein every layer comprises the samecompound.
 12. The polymer film of claim 1, wherein the film comprisesfrom two to ten layers.
 13. The polymer film of claim 1, wherein thefilm comprises from two to four layers.
 14. The polymer film of claim 1,wherein the film has a conductivity of at least about 100 S/cm and atransparency of at least about 80%.
 15. The polymer film of claim 1,wherein the thickness of the polymer film is no more than about 1 μm.16. A display device comprising the polymer film of claim 1.